A coaxial magnetron is a thermionic vacuum tube device having a cathode coaxially disposed with respect to an associated anode. Electrons are emitted by the heated cathode and are accelerated toward the anode by an electric field created by a high voltage applied between the cathode and the anode. The interelectrode space is traversed by a magnetic field created by externally disposed permanent magnets or electromagnets. The magnetic field acts transversely to the cathode-to-anode path such that the emitted electrons are deflected by the field. When properly connected to a resonant line, the magnetron tube can operate as an oscillator and be made to generate frequencies measured in thousands of megaHertz and having associated wavelengths of a few centimeters.
The output frequencies of magnetron tubes depend on a number of factors ranging from mechanical variations incurred during their manufacture to environmental variations occurring during their subsequent use. Since some magnetron tube applications require fixed- frequency operation and other applications require specifically controlled, variable-frequency or multiple-frequency operation, provision must be made for tuning the tubes.
The conventional method for controlling the frequency of a mechanically tuned coaxial magnetron is by operably coupling a position servo control loop to a moveable tuning member. Since specified frequencies must be set and maintained under a wide variety of environmental conditions, a number of problems attend this method.
One major problem arises from the fact that the tuning member position associated with a particular output frequency of one magnetron tube may not provide the same output frequency if another tube is substituted for the original. A possible solution to this problem would be to incorporate a data storage module in the assembly of each magnetron tube to store the characteristics of that tube so that, when the tube is substituted for another, the data could be used to compute the tuning member position for a desired output frequency. Locating a data storage module, comprising relatively low voltage digital circuits, near the high voltage and current transients attending an operating magnetron tube would invite additional problems, however.
Another major problem results from the temperature sensitivity of the tuner and of the associated tuning member position sensing device, and this problem is often addressed by placing critical components in constant-temperature ovens. Such ovens, however, must be operated at temperatures above the highest anticipated ambient temperature. Because the components must be closely coupled to the magnetron, which itself operates at substantially elevated temperatures, a significant reduction in reliability is unavoidable.
Additionally, some critical components, notably the tuning member, which is located inside the vacuum envelope of the magnetron tube, cannot be placed in an oven. Therefore, one or more temperature sensors must be located proximate these critical components and the outputs of the sensors used in conjunction with an error-correcting function to compensate for temperature caused frequency deviations. Since error-correcting functions will be somewhat different for each tube, a technique such as that previously described using stored tube characteristics, with its attending problems, must then be used to attempt to compensate for the differences in tube characteristics.
Another problem involves frequency pulling resulting from radio frequency load variations. An effective solution to this would be to insert a high-power ferrite load isolator between the magnetron and the antenna. The isolator would, of course, incur a radio frequency power loss; and an increase in magnetron output power would be required to compensate for this loss. This would require an increase in modulator output power, which, in turn, would require a greater power output from the high-voltage supply. All of this would require a greater cooling system capacity, which would consume even more power. Overall, this would result in a substantial reduction in system efficiency and significantly increase the size and cost of the system.
Yet another problem results from the long-term effects of component aging. The solution to this problem would be the periodic calibration and adjustment of electrical and mechanical trimmers, likely requiring specially trained personnel using special test equipment.
The present invention comprises an effective combination of elements that at once maximizes the speed, effectiveness and efficiency of controlling the frequency of a mechanically tuned coaxial magnetron while minimizing the cost, size and maintenance of the frequency control system.